Crown-Ether-Like PbII-Metal Framework with Dual- and Bimodal Emissive Properties Based on Its Photochromic Precursor by Leaching

Dual Emission in the Near-Infrared and Visible Regions from a Mixed Cyanido-Bridged EuIII/NdIII(4-OHpy)-CoIII Layered Material

Coordination polymers (CPs) with a dual emission spanning from the visible (vis) to near-infrared (NIR) regions of the electromagnetic spectrum are used for optical sensors, medical diagnostics, and telecommunication technologies. We herein report the synthesis, structural characterization, and optical response of heterometallic cyanido-bridged layered {[Eu_xNd_y(4-OHpy)₂(H₂O)₃][Co(CN)₆]} CPs, where 4-OHpy = 4-hydroxypyridine, with a multicolor emission profile across the vis and NIR regions. The crystals show an efficient energy transfer (ET) from the 4-OHpy ligand and the [Co(CN)₆] ions to the Eu³⁺ and Nd³⁺ ions, resulting in an enhanced photoluminescence (PL) efficiency. We study the ET with steady-state and time-resolved PL, reporting an ET between the Ln³⁺ centers. The excitation-dependent emission of the mixed Ln³⁺ CPs and the control over the PL lifetime yield new insights into the optoelectronic properties of these materials.

Organo-macrocycle-containing hierarchical metal-organic frameworks and cages: design, structures, and applications

Developing hierarchical ordered systems is challenging. Using organo-macrocycles to construct metal-organic frameworks (MOFs) and porous coordination cages (PCCs) provides an efficient way to obtain hierarchical assemblies. Macrocycles, such as crown ethers, cyclodextrins, calixarenes, cucurbiturils, and pillararenes, can be incorporated within MOFs/PCCs and they also endow the resultant composites with enhanced properties and functionalities. This review summarizes recent developments of organo-macrocycle-containing hierarchical MOFs/PCCs, emphasizing applications and structure-property relationships of these hierarchically porous materials. This review provides insights for future research on hierarchical self-assembly using macrocycles as building blocks and functional ligands to extend the applications of the composites.

Triazine Poly(carboxylic acid) Metal-Organic Frameworks and the Fluorescent Response with Lead Oxygen Clusters: [Pb7(COO)12X2] by Halogen Tuning (X = Cl, Br, or I)

A series of novel metal-organic frameworks were synthesized by using semirigid ligand 1,1',1″-(1,3,5-triazine-2,4,6-triyl)tripiperidine-4-carboxylic acid (H₃TTPCA) and lead halide (Cl, Br, or I). The three complexes were characterized by elemental analysis, infrared spectroscopy, ultraviolet-visible spectroscopy, powder X-ray diffraction analysis, and thermogravimetric analysis. X-ray single-crystal diffraction analysis demonstrated that all three complexes were three-dimensional inorganic-organic framework structures with Pb-X₂ (X = Cl, Br, or I). However, slight differences in the chemical environment were the focus of the coordinated halogen atoms and the different compositions of metal oxygen clusters: [Pb₇(COO)₁₂Cl₂], [Pb₇(COO)₁₂Br₂], and [Pb₇(COO)₁₂I₂]. Because of the fluorescence of the organic ligand, the three complexes showed similar photoluminescence properties at room temperature, but the intensity of emissions decreased gradually with an increase in the atomic radius of coordinated halogen atoms. Interestingly, in the fluorescence response tests, complexes 1 and 2 displayed an optical signal of fluorescence "turn-on" while complex 3 showed an optical signal of fluorescence "turn-off". Here we aim to provide a possible mechanism to explain these unique and contradictory luminescence results.

Fluorescent "Turn-on" Sensing Based on Metal-Organic Frameworks (MOFs)

Metal-organic frameworks (MOFs) have evolved as an exciting class of materials in the domain of porous materials. The unique features of these materials arise from the combined properties of metal ions/clusters and organic struts which form the building blocks of these fascinating architectures. Among other multifarious applications, MOFs have shown tremendous applications as sensory materials for a wide variety of species. The signal transduction induced mechanism in these confined nanospaces generate optical output in response to a particular analyte which can be detected by wide variety of detection techniques. Fluorometric methods of sensing is one of widely studied method over past few decades. MOF-based fluorometric detection is a key research theme developed over the past few years. In this review, we give a brief overview of the recent developments of MOFs as "turn-on" sensors for a wide range of analytes (viz. cations, anions, volatile organic compounds (VOCs), etc.).

A stable luminescent zinc–organic framework as a dual-sensor toward Cu2+ and Cr2O72−, and excellent platform-encapsulated Ln3+ for systematic color tuning and white-light emission

A stable three-fold interpenetrated framework (Zn-MOF), showing chemical sensing for Cu²⁺ and Cr₂O₇²⁻, tunable luminescence and white-light emission after encapsulating lanthanide cations, was synthesized and characterized.

Single Component Lanthanide Hybrids Based on Metal-Organic Framework for Near-Ultraviolet White Light LED

Near-UV single-phase white-light phosphor (Eu0.045Tb0.955CPOMBA/La0.6Eu0.1Tb0.3CPOMBA) based on metal-organic framework was prepared by in situ doping isostructural lanthanide MOF with Eu(3+) and Tb(3+), and it is found that the energy can effectively transfer from organic ligand to lanthanides, which can overcome weak absorption under direct excitation of lanthanide ions due to the forbidden f-f transitions. The photoluminescence and thermostability of the new MOF phosphor are investigated, and effective white-light emission is achieved under 365 and 380 nm excitations. By employing Eu0.045Tb0.955CPOMBA as phosphor, we fabricated a near-ultraviolet white-light-emitting diode (n-UV WLED) (365 nm) with low CCT (5733 K), high CRI (Ra = 73.4), and CIE chromaticity coordinate (0.3264, 0.3427). This approach may open new perspectives for developing single-phase UV phosphors.

Chemical Sensors Based on Metal-Organic Frameworks

Metal-organic frameworks (MOFs) as chemical sensors have developed rapidly in recent years. There have been many papers concerning this field and interest is still growing. The reason is that the specific merits of MOFs can be utilized to enhance sensitivity and selectivity by various energy/charge transfers occurring among different ligands, ligand, and metal centers, such as from ligands to metal centers or metal centers to ligands, as well as from MOF skeletons to guest species. This review intends to provide an update on recent progress in various applications of different MOF-based sensors on the basis of their luminescent and electrochemical responses towards small molecules, gas molecules, ions (cations and anions), pH, humidity, temperature, and biomolecules. MOF-based sensors function by utilizing different mechanisms, including luminescent responses of "turn-on" and "turn-off", as well as electrochemical responses.

Calix-Like Metal-Organic Complex for High-Sensitivity X-Ray-Induced Photochromism

Metal-organic complexes (MOCs) as promising candidates for directly visual X-ray detection at room temperature are rare and discovered unexpectedly, even though every crystalline material needs X-ray diffraction studies. Here, we report a rational strategy of mimicking host-guest system for developing high-sensitive X-ray-induced photochromic materials. Two resulting calix-like metal-organic complexes (cMOC-1 and cMOC-2) were prepared by encapsulating the electron-capturing "guest" into the cavity of calix-like electron-donating "host." One of them (cMOC-1) achieves instantaneous X-ray-induced photochromism and easy recovery by synergizing the aprotic matrix of MOC and similar host-guest interaction. Their strikingly different response to X-ray irradiation resulting from the structural difference demonstrates the feasibility and acceptability of our design strategy. This strategy may open new perspectives for developing high-performance photo-responsive functional materials.

Stabilizing and color tuning pyrazine radicals by coordination for photochromism

This work reports the first photochromic pyrazine-based compound that can undergo photoinduced charge separation and yield stable pyrazine radicals.

An electron-transfer photochromic metal–organic framework (MOF) compound with a long-lived charge-separated state and high-contrast photoswitchable luminescence

A new photochromic MOF compound exhibits a charge-separated state with lifetime exceeding the reported values of the analogues and a luminescence contrast higher than those of most known pyridine derivative-based photochromic compounds.

Layer-structured coordination polymers based on 5-(1H-tetrazol-5-yl)isophthalic acid: structure, sensitization of lanthanide(iii) cations and small-molecule sensing

Compound 1 can serve as a host for encapsulation of Ln³⁺ ions.

An Ideal Detector Composed of Two-Dimensional Cd(II)-Triazole Frameworks for Nitro-Compound Explosives and Potassium Dichromate

The two-dimensional (2D) metal-organic framework (MOF) [Cd(TPTZ)(H2O)2(HCOOH)(IPA)2]n (1; TPTZ = {4-[4-(1H-1,2,4-triazol-1-yl)phenyl]phenyl}-1H-1,2,4-triazole, IPA = isophthalic acid) has been constructed with the π-electron-rich aromatic ligand TPTZ, auxiliary ligand IPA, and the metal Cd(2+) ion with a d(10) configuration under solvothermal conditions. Complex 1 exhibits a strong ligand-originated photoluminescence emission, which is selectively sensitive toward electron-deficient nitroaromatic compounds, such as nitrobenzene (NB), 1,3-dinitrobenzene (m-DNB), and 1,4-dinitrobenzene (p-DNB), and nitro-aliphatic compounds, such as nitromethane (NM) and tris(hydroxymethyl)nitromethane. This property makes complex 1 a potential fluorescence sensor for these chemicals. Single-crystal X-ray diffraction studies revealed that dinuclear cadmium building units were further bridged by TPTZ ligands to give a four-connected uninodal net with the Schläfli symbol of [4.6(3).4.6(3).6(2).6(4)].

A Series of Multifunctional Metal-Organic Frameworks Showing Excellent Luminescent Sensing, Sensitization, and Adsorbent Abilities

A series of highly luminescent-active metal-organic frameworks (MOFs) 1-3 with hierarchical pores have been rationally constructed and fully characterized. The predesigned semi-rigid hexacarboxylate ligand hexa[4-(carboxyphenyl)oxamethyl]-3-oxapentane acid (H6 L) has been adapted with various space-directed N donors (i.e., 2,2'-bipyridine, 4,4'-di(1H-imidazol-1-yl)-1,1'-biphenyl, and 1,3,5-tri(1H-imidazol-1-yl)benzene) from a bidentate V-shape to a tridentate Y-shape. This family of multifunctional MOF materials represents a variety of potential applications in the following aspects: first, as luminescent sensors that show a fast and sensitive detection for pollutant CrO4 (2-) and Cr2 O7 (2-) ions in aqueous media; second, as adsorbents that can rapidly remove harmful organic dyes; third, as an antenna that can effectively sensitize visible-light-emitting Tb(3+) ions. These multifunctional MOF materials combine optical-sensing, adsorption, and sensitization properties, thus are very useful in many potential applications. Furthermore, these materials have proved to be reusable.

Cation-Exchange Porosity Tuning in a Dynamic 4d-4f-3d Framework for Ni(II) Ion-Selective Luminescent Probe

A heterometallic complex {[Yb2(L)6Cd2][Cd(H2O)6]·6H2O}n (Yb-Cd) (H2L = oxidiacetic acid) was synthesized under hydrothermal conditions. In Yb-Cd, each L chelates to one Yb(3+) center and bonds to two Cd(2+) ions in an anti-anti configuration. Yb and Cd atoms are arrayed alternatively and connected by O-C-O bridges to form a cubic octahedral cage as the secondary building unit. Consequently, topological NaCl nets with high symmetry in the cubic space group Fd-3c have been constructed. The [Cd(H2O)6](2+) moieties lying in the porosity of anionic metal-organic framework (MOF) act as the thermodynamically stable species, required to balance the two negative charges of [Yb2(L)6Cd2](2-) in Yb-Cd. Interestingly, when Yb-Cd was employed as a precursor and emerged in the aqueous solution of Mn(ClO4)2·6H2O or Zn(ClO4)2·6H2O, a reversible single-crystal-to-single-crystal transformation process driven by [Cd(H2O)6](2+) cations has been exhibited to generate the heterotrimetallic coordination polymer {[Yb2(L)6Cd2][Mn(H2O)6]·6H2O}n (Yb-Cd-Mn) or {[Yb2(L)6Cd2][Zn(H2O)6]·6H2O}n (Yb-Cd-Zn). To the best of out knowledge, Yb-Cd-Mn and Yb-Cd-Zn are the first examples representing 4d-4f-3d polymers based on multicarboxylic acid. Luminescent studies reveal that Yb-Cd-Zn may serve as a good candidate of Ni(2+) a luminescent probe. To our knowledge, Yb-Cd-Zn represent the fist example of the 4d-4f-3d framework to exhibit luminescent selectivity for Ni(2+).

A blue luminescent MOF as a rapid turn-off/turn-on detector for H2O, O2and CH2Cl2, MeCN: 3∞[Ce(Im)3ImH]·ImH

A blue emitting MOF is presented as a fast turn-off/turn-on detector for various analytes utilising 5d–4f-transitions of Ce³⁺.

Intercalation of lanthanide cations to a layer-like metal–organic framework for color tunable white light emission

MIL-124 has been synthesized by solvothermal approach, and the luminescence was tuned by doping Ln³⁺ into the channels. With adjustment of the excitation wavelength and concentration of Ln³⁺, dichromatic (MIL-124@Eu³⁺) and trichromatic (MIL-124@Eu³⁺/Tb³⁺) white-light-emission can be achieved.

Dual emission tunable in the near-infrared (NIR) and visible (VIS) spectral range by mix-LnMOF

In this study, we describe the synthetic approach, crystallographic structure, luminescent behavior in heteronuclear coordination polymers with emission in the visible (Eu³⁺ and organic ligand) and near-infrared (Nd³⁺) range.

Metal-organic frameworks as sensory materials and imaging agents

Metal-organic frameworks (MOFs) are a class of hybrid materials self-assembled from organic bridging ligands and metal ion/cluster connecting points. The combination of a variety of organic linkers, metal ions/clusters, and structural motifs can lead to an infinite array of new materials with interesting properties for many applications. In this Forum Article, we discuss the design and applications of MOFs in chemical sensing and biological imaging. The first half of this article focuses on the development of MOFs as chemical sensors by highlighting how unique attributes of MOFs can be utilized to enhance sensitivity and selectivity. We also discuss some of the issues that need to be addressed in order to develop practically useful MOF sensors. The second half of this article focuses on the design and applications of nanoscale MOFs (NMOFs) as imaging contrast agents. NMOFs possess several interesting attributes, such as high cargo loading capacity, ease of postmodification, tunable size and shape, and intrinsic biodegradability, to make them excellent candidates as imaging contrast agents. We discuss the use of representative NMOFs in magnetic resonance imaging (MRI), X-ray computed tomography (CT), and optical imaging. Although still in their infancy, we believe that the compositional tunability and mild synthetic conditions of NMOF imaging agents should greatly facilitate their further development for clinical translation.

Tunable colors and white-light emission based on a microporous luminescent Zn(ii)-MOF

A microporous MOF material (JUC-113) was synthesized, which has permanent porosity and emits blue light. According to three-primary color, the luminescent properties of JUC-113 can be easily tuned by different combinations of the encapsulation amount of Tb³⁺ and Eu³⁺ and generated white-light emission materials.

The structures of three Hg2X4L2macrocycles {X= Cl, Br and I, andL= 1,2-bis[4-(pyridin-3-yl)phenoxy]ethane} assembled from ether-bridged dipyridyl ligands

The new ether-bridged dipyridyl ligand 1,2-bis[4-(pyridin-3-yl)phenoxy]ethane (L) has been used to synthesize three isostructural centrosymmetric binuclear HgIImacrocycles, namely bis{μ-1,2-bis[4-(pyridin-3-yl)phenoxy]ethane-κ2N:N′}bis[dichloridomercury(II)], [Hg2Cl4(C24H20N2O2)2], and the bromido, [Hg2Br4(C24H20N2O2)2], and iodido, [Hg2I4(C24H20N2O2)2], analogues. The Hg atoms adopt a highly distorted tetrahedral coordination environment consisting of two halides and two pyridineN-donor atoms from two bridging ligands. In the solid state, the macrocycles form two-dimensional sheets in thebcplane through noncovalent Hg...XandX...X(X= Cl, Br and I) interactions.

Two threefold interpenetrated two-dimensional frameworks based on a flexible imidazole-containing ligand and benzene-1,4-dicarboxylate

The open-chain polyether-bridged flexible ligand 1,2-bis[2-(1H-1,3-imidazol-1-ylmethyl)phenoxy]ethane (L) has been used to create two two-dimensional coordination polymers under hydrothermal reaction of L with Cd(II) or Co(II), in the presence of benzene-1,4-dicarboxylic acid (H(2)bdc). In poly[[(μ(2)-benzene-1,4-dicarboxylato){μ-1,2-bis[2-(1H-1,3-imidazol-1-ylmethyl)phenoxy]ethane}cadmium(II)] dihydrate], {[Cd(C(8)H(4)O(4))(C(22)H(22)N(4)O(2))]·2H(2)O}(n), (I), and the cobalt(II) analogue {[Co(C(8)H(4)O(4))(C(22)H(22)N(4)O(2))]·2H(2)O}(n), (II), the Cd(II) and Co(II) cations are six-coordinated by four carboxylate O atoms from two different bdc(2-) dianions in a chelating mode and two N atoms from two distinct L ligands. The metal ions, bdc(2-) dianions and L ligands each sit across crystallographic twofold axes. The bdc(2-) coordination mode and the coordinating orientation of the L ligand play an important role in constructing the novel two-dimensional framework. Complexes (I) and (II) are threefold interpenetrated two-dimensional frameworks; their structures are almost isomorphous, while the bond lengths, angles and hydrogen bonds are different in (I) and (II).

Luminescent multifunctional lanthanides-based metal-organic frameworks

Metal-organic frameworks based on trivalent lanthanides (LnMOFs) are a very promising class of materials for addressing the challenges in engineering of luminescent centres. Lanthanide-bearing phosphors find numerous applications in lighting, optical communications, photonics and biomedical devices. In this critical review we discuss the potential of LnMOFs as multifunctional systems, which combine light emission with properties such as microporosity, magnetism, chirality, molecule and ion sensing, catalysis and activity as multimodal imaging contrast agents. We argue that these materials present a unique chance of observing synergy between several of these properties, such as the coupling between photoluminescence and magnetism. Moreover, an integrated approach towards the design of efficient, stable, cheap, environmentally-friendly and multifunctional luminescent LnMOFs is still missing. Although research into LnMOFs is at its early stage and much basic knowledge is still needed, the field is ripe for new ideas, which will enable sensor devices and photonic prototypes to become a commercial reality (81 references).